1. Field of the Invention
The present invention is directed to coating compositions having long pot lives and short dry times under ambient conditions in which the binder is based on polyisocyanates containing allophanate groups and aldimines prepared from diamino dicyclohexyl methane.
2. Description of the Prior Art
Coating compositions which may be cured at room temperature are known. One-component coating compositions contain fully reacted polyurethanes as the binder. These compositions have the advantage that they are available as fully formulated systems which may be directly applied to suitable substrates without any preliminary steps except for mild stirring. Disadvantages of these systems are that large amounts of organic solvents are needed to reduce the viscosity of fully reacted, i.e., high molecular weight, polyurethanes. The coating compositions are cured by evaporation of the solvent which is objectionable from an environmental perspective. In addition, in order to solubilize the polyurethanes in organic solvents, they must be essentially linear polyurethanes. While such polyurethanes possess properties which are suitable for many applications, they do not provide certain properties, e.g., solvent resistance, which may be obtained from crosslinked polyurethanes.
Two-component coating compositions are also known. These compositions come in two containers. The first contains a polyisocyanate, while the second contains an isocyanate-reactive component, generally a polyol. The components are not mixed until they are ready to be used. One advantage of these compositions is that because the components are not pre-reacted to form a high molecular weight polymer, a suitable processing viscosity can be achieved without the need for large amounts of organic solvents. In addition, higher functional components can be used to obtain highly crosslinked coatings which possess properties which surpass those possessed by one-component coatings.
The disadvantages of these compositions is that they cannot be applied without a preliminary mixing step in which it is critical that the components are mixed in the right proportions. Special metering and mixing equipment is needed to conduct this process on a commercial scale. If the components are mixed in the wrong proportions, then the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they must be used in a timely fashion. If not, they continue to react until an unusable solid is finally obtained.
It is an object of the present invention to provide coating compositions which possess the advantages of the known one- and two-component coating compositions without possessing any of their disadvantages. This object may be achieved with the coating compositions according to the present invention wherein the binder is based on a polyisocyanate containing allophanate groups and an aldimine prepared from diamino dicyclohexyl methane. The advantages of these coating compositions are as follows:
1) They may be prepared as fully formulated systems in a manner similar to the one-component systems discussed above and do not require the special metering and mixing equipment required to formulate and mix two-component systems. PA0 2) Because the components are not pre-reacted to form high molecular weight polymers, large amounts of solvents are not necessary to reduce the viscosity of the coating compositions. PA0 3) Highly functional resins can be used to provide crosslinked coatings with exceptional properties. PA0 4) The manufacturer can accurately formulate the compositions such that there is no need for on-site mixing and metering equipment. PA0 5) Because of the excellent compatibility between the components, which is disclosed in copending application, Attorney's Docket No. Mo-4053, coatings prepared from the compositions possess excellent optical properties. PA0 6) After the polyisocyanates and aldimines are mixed, they have long pot lives when compared at equivalent solids to coating compositions based on other aldimines, and yet rapidly cure under ambient conditions to form coatings when applied to substrates. PA0 a) polyisocyanate component containing PA0 b) an aldimine based on the reaction product of diamino dicyclohexyl methane with an aldehyde corresponding to the formula: EQU O=CHCH(R.sub.1)(R.sub.2)
This last advantage of the compositions according to the invention is particularly surprising and unexpected because the known high solids coating compositions, which cure rapidly when applied to substrates, do not possess long pot lives. On the other hand, compositions which remain stable in storage (e.g., by blocking the polyisocyanate), do not cure rapidly when applied to substrates. Accordingly, it could not be expected that it would be possible to provide coating compositions which satisfy the requirements of storage stability and rapid cure under ambient conditions.
U.S. Pat. No. 3,420,800 and 3,567,692 disclose coating compositions containing polyisocyanates and either aldimines or ketimines. However, the coating compositions described in these references do not possess long pot lives, i.e., good storage stability, nor are the components compatible with each other resulting in unacceptable coatings. These references did not recognize the specific binder components described in accordance with the present invention, which necessary to obtain long pot lives and rapid cure.